Would I be able to record vocals and guitar simultaneously with mics in the front inputs and TRS cables from my guitar effects pedal into the rear line level inputs (stereo signal)?
Depends on your effects pedal. Please clarify brand/model of your pedal in order to get definitive answer.
You can connect headphone output from your ZOOM to UR24’s inputs 3&4 using TRS to 2xTS cable. Like these ones:
Just be careful with levels. Headphone output may have high output level which can overload UR’s input, even
though ZOOM’s technical specifications seem to indicate that this shouldn’t be a problem. If this happens just
turn ZOOM’s volume down.